a. Field of the Invention
The present invention relates to a test receiver unit for receiving and testing a signal transmitted over a communications link, and to a communications system and method that uses a test receiver unit to characterise distortion in a the signal. The signal may be an electrical signal transmitted over an electrical communications link, or an optical signal transmitted over an optical communications link.
b. Related Art
A transmitted signal may be subject to numerous sources of noise and distortion, both in the generation of the signal and its transmission through a communications channel. For example, sources of noise in an optical transmitter or receiver include thermal noise and shot noise. A receiver section of the receiver may also use an avalanche photodiode for high sensitivity, but this will introduce avalanche photodiode noise. The generation of an optical signal will in general be subject to other sources of noise or drift. For example, the output power of a laser diode will be subject to slow drift as it heats up in use or from changes in ambient temperature. Laser diodes and other optical components such as gratings are also very sensitive to optical feedback from back reflections. Such feedback will cause rapid changes in system distortion, which can close the eye pattern in a received signal. In this case the system suffers from intersymbol interference (ISI), which in conjunction with system noise, increases the bit error rate (BER) of the received signal. ISI is interference resulting from the data pattern itself. Such effects may be due to rapid heating or other non-linear or linear effects within the laser source itself, and/or may be due to non-linear variations owing to the use of new or existing multimode optical fibre in an optical communications link. In a multimode optical fibre, different modes have different propagation velocities, which tend to disperse a pulse into adjacent pulses, thereby causing ISI. Pulse dispersion also occurs in single mode fibre, but to a lesser extent. Such effects will therefore tend to close an eye pattern and increase the measured BER at the receiver.
Similarly, electrical signals transmitted in electrical communications links are also subject to sources of noise.
High-speed optical communications links, for example links operating at a data rate of at least 5 Gbit/s, have tended to use single mode optical fibre together with high precision optical fibre connectors. This has been the case even when such links are operating over short distances, such as in local area networks where links are typically of the order of 10 m to 100 m in length, or over medium distances, such as in metro networks where links are typically of the order of 1 km to 10 km in length. While such high speed communications links provide reliable performance at a very low BER, for example 10−12, there is a need for comparable performance at greatly reduced cost, and in practice this requires the use of multimode optical fibre and cheaper connectors, and preferably also cheaper and potentially less stable sources of optical radiation.
It has also been proposed to use an equaliser circuit to compensate for intersymbol interference, both in receivers for optical signals and receivers for electrical signals. In the case of an optical receiver, such an equaliser circuit receives as an input the output from a photoreceiver including a photodetector circuit, and then generates from this at least two equaliser coefficients. A signal delay line also receives the output from the photodetector circuit. Tapped outputs from the delay line are each multiplied or otherwise combined with one of the equaliser coefficients, and then summed together to generate an equalised output signal. Equalisers in a receiver for electrical signals have a similar structure.
One common approach is to use a least mean squared (LMS) algorithm to generate the equaliser coefficients. Such prior art equalisers use an iterative approach that aims to converge on the correct equaliser coefficients slowly over many hundreds or thousands of repeat calculations. Although it may be possible to achieve higher performance at increased cost and complexity in the equaliser, all such prior art approaches suffer from the limitation that the repetitive and iterative method of calculating the equaliser coefficients imposes a major burden on the complexity of the circuitry and software inside the receiver, and hence also on electrical power consumption by the receiver and consequent cooling requirements for the receiver. These problems are compounded at data rates in excess of about 1 Gbit/s, because of the difficulty and expense of increasing the speed of the equaliser calculations.
For some types of optical receiver and transceiver units, there are de facto industry standards on the total maximum allowable electrical power consumption. In particular, the Small Form-Factor Pluggable (SFP) Transceiver Multisource Agreement (MSA), which includes transceivers with transmission rates up to 5 Gbit/sec, operating over single mode and multimode fibre, specifies a maximum electrical power consumption of 1 W. Several other MSA's e.g. XFP, SFF, Gbic, Xenpak, Xpak, and X2, specify varying levels of electrical power consumption. Such standards are necessary to maintain interchangeability between similar components manufactured by different sources. There are also industry standards on the package size and configuration of such components, necessary to ensure that similar components from different manufacturers are plug-compatible. Such physical constraints limit the amount of passive cooling that may be afforded by heat sinks or cooling fins.
Receivers for electrical signals may suffer from similar limitations.